Heterogeneous deployments can be described as a deployment of a radio access network, wherein low power nodes are placed throughout a macro-cell layout. The low-power nodes might be so-called pico cells or so-called Closed Subscribe Group (CSG) cells. Such a placement of pico (or CSG) cells might e.g. provide enhanced capacity locally or improved indoor coverage.
Within a deployment of the pico cells, certain subframes are provided, in which a macro cell significantly or almost entirely reduces transmission energy; such subframes are often referred to as Almost-Blank Subframes (ABS). A deployment of macro- and pico cells requires time synchronization (time alignment) and a proper planning of an ABS scheme.
In above described heterogeneous networks, in certain areas, the UE shall be able to connect or stay connected to the pico cell although the macro sell signals, e.g. a so-called cell-specific reference signal -CRS- from the macro cell is received with higher power compared to a corresponding pico cell CRS Such deployment is also being referred to as cell range extension (CRE) being specified in recent 3GPP LTE or Evolved Universal Terrestrial Radio Access (E-UTRA) specification documents. Thereto, documents 3GPP TS 36.300 version 10.0.0 (as member of 3GPP LTE Release 10 documents, also being referred to as LTE Rel-10 in the following) and 3GPP TS 36.300 version 11.0.0 (as member of 3GPP LTE Release 11 documents also being referred to as LTE Rel-11 in the following) each specify (e.g. in section 16.1.5) an inter-cell coordination function located in the eNodeB -eNB- to manage radio resources for keeping inter-cell interference under control.
The macro cell CRS thus may be received with higher power as compared to the pico cell CRS, e.g. about 0-6 dB according to a so-called enhanced intercell interference coordination -eICIC- according to LTE Rel-10, and 6-12 dB according to a so-called further enhanced intercell interference coordination -feICIC- according to LTE Rel-11.
FIG. 1 shows a sketch of an exemplary simple heterogeneous network comprising a macro base station or macro eNB 11, a macro cell (area) 12 served by the macro eNB, a pico base station or pico eNB 21, a pico cell (area) served by the pico eNB 21, wherein the pico cell area is divided into a kernel pico cell area 22a and an extended pico area 22b, in the following also being referred to as cell range extension -CRE- area. Further, a first exemplary user equipment UE1 actually located in the kernel pico cell area 22a and a second user equipment UE2 actually located in the CRE area 22b is depicted.
To secure reliable transmission of the control channel and efficient transmission of the Physical downlink shared channel -PDSCH- of the second user equipment UE2 located in the CRE of the pico cell 22b, ABS subframes in the macro cell 21 are configured to transmit only limited (or necessary) data, e.g. only signal such as Physical Broadcast Channel -PBCH- signals, Primary/Secondary Synchronization Signals -PSS/SSS- and CRS according to in Rel-10. Hence, in ABS subframes, the second user equipment UE2 experiences low interference from the macro cell for the data channel, and conversely high interference from macro cells in non-ABS subframes. On the other hand, first user equipments located sufficiently close to the center of the pico cell, also being referred to as non-CRE UEs, e.g. the first user equipment UE1 according to FIG. 1, the interference from the macro cell may be relatively small as compared with the signal from the pico cell.
One of the properties of above-discussed releases is that it allows the eNB to restrict the channel measurements by UEs attached to them to a specific set of ABS subframes. The UEs are provided with different CSI-measurement subsets corresponding to the subframes that the UE is allowed to use. CRE users, i.e. user equipments located in the CRE, are only allowed to report CSI measurements for the ABS as they are only allowed to transmit during these subframes. Non-CRE users may transmit 2 different subsets of the CSI measurements, one for the ABS and another for the non-ABS as they are may be allowed to transmit through all the subframes.
FIG. 2 shows an exemplary pattern of ABS subframes within a plurality of subsequent subframes S1-S12. By way of example every fourth subframe, e.g. subframes S2, S6 and S10 of the consecutive subframes S1-S12 are ABS frames.
The channel state information (CSI corresponding to CQI/PMI/RI) is different for ABS and non-ABS subframes. Averaging the CSI over both ABS and non-ABS subframes is not meaningful and, hence, e.g. according to Rel-10, two subframe sets, CSI_0 and CSI_1, are signaled to the UE for measurement, as illustrated in FIG. 2. The UE currently located within the CRE area, e.g. UE1 according to FIG. 1 may then perform an independent measurement and a corresponding feedback for each CSI subframe set CSI_0 and CSI_1.
UEs perform an automatic gain control -AGC- to normalize the input signal such that the limited dynamic range of the subsequent processing subsystems causes minimal degradation of the signal quality. Typically an AGC is set to provide some average magnitude value that is below the maximum value of a corresponding analog-to digital circuit ADC to avoid a signal clipping. An optimal gain of AGC may be derived e.g. from a measured variance of the reference signal or from the measured mean of the absolute value of the reference signal. These measurements are usually identical for both quadrature components of the equivalent complex baseband signal and can be averaged over both quadrature components. The gain is usually adjusted to maximize a average signal-to-quantization noise ratio (SNR), whereby quantization noise corresponds to noise from clipping or saturation, NClip, as well as noise due to the limited resolution of the quantized signal, Nquant.
For maximizing the signal-to-quantization noise ratio, there are two conflicting requirements. On the one hand, the signals should be kept large with respect to the quantizing interval to achieve a good resolution. On the other hand, the signal should be kept small enough to avoid (excessive) saturation or clipping in the quantizer. The opposing requirements may be resolved by scaling the input signal so that its root mean squared (RMS) value is a specified fraction of the full-scale quantizer range. The specified fraction is chosen to balance the saturation errors (weighted by their probability of occurrences), also called clipping errors, against the quantizing errors (which are similarly weighted) and thus achieve a maximum SNR. The relationship among SNR, quantizing error and clipping error is shown in FIG. 3 for Gaussian distributed signals, where SNR=(S+I)/(Nclip+Nquant). Here, NClip is the variance of clipping error, Nquant is the variance of quantizing error, S is the desired signal power, and I represents interference and thermal noise power received in the front-end. FIG. 3 by way of example shows the SNR as a function of the ratio of ADC adjustment in dB relative to full scale (dBFS) of the ADC when 10 bits are used for quantizing and the input signal is Gaussian distributed.
FIG. 3 shows a diagram of an exemplary ADC dynamic range versus noise without DC offset. The achievable dynamic range or (S+I)/(Nclip+Nquant) is a function of the target value in dBFS, whereby exemplarily the input signal is Gaussian distributed and quantized with 10 bits (with no DC offset). It can be seen that reduced signal levels correspond to smaller AGC adjustment on the abscissa, and represent a shift to the left. Similarly, increased input signal levels also correspond to larger AGC adjustment on the abscissa, and represent shift to the right. According to the example of FIG. 3, a maximum of SNR=51.56 dB can be achieved when AGC adjustment is −13.04 dB of full scale in the case of Gaussian distributed signals quantized with 10 bits per quadrature component.
The gain for AGC is usually adjusted by comparing a certain measured power (e.g. a measured mean absolute amplitude) with a corresponding pre-configured target value for the power (or target value for the mean absolute amplitude) that depends on the chosen quantization of the signal flow as described in the previous Section. AGC adjustment may be based on following power measurements:                Reference signal resource element received power -RSRERP- measurements indicative of a received power of the serving cell reference signal (RS) resource elements (REs),        Reference signal received power -RSRP- measurements indicative of the reference signal resource element received power, wherein some disturbing power, e.g. power with respect to interference (e.g. being removed by cancellation or filtering), thermal noise (hence RSRERP=RSRP+interference power+thermal noise power at the reference signal resource elements)        Received signal strength indicator -RSSI- measurements indicative of a linear average of the total received power observed only in OFDM symbols containing reference symbols (e.g. for one of a plurality of antenna ports, e.g. antenna port 0), in the measurement bandwidth, from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise etc.        
When enhanced intercell interference coordination -eICIC- is applied according to Rel-10 or further enhanced intercell interference coordination -feICIC- is applied according to Rel-11, the total received power may change significantly between ABS subframes and non-ABS subframes. Even in ABS subframes itself, the total received power observed in each OFDM symbols may also change significantly. The received power observed in each OFDM symbols in the measurement bandwidth.
Conventional automatic gain control -AGC- algorithms make no difference between ABS and non-ABS subframes. Even in case of ABS, the CRS may be still transmitted (to maintain backward compatibility). Thus interference due to CRS transmission may still be significant. It is one aim of the invention to improve the function of the AGC by taking into account the existence ABS subframes.